Redox covalent attachment

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

In an attempt to establish unequivocally the spectroscopic features of coordinated (vs uncoordinated) phenoxyl radicals a series of phenolato precursor complexes containing a spectroscopically and redox-innocent Ga(III), Sc(III), or Zn(II) central metal ion were synthesized (142-148). In order to avoid metal-ligand bond dissociation in solution, the phenolate or, after one-electron oxidation, phenoxyl moieties were covalently attached to the strongly metal ion binding 1,4,7-triazacy-clononane (149) backbone. Thus a series of phenolate pendent-arm macrocyclic... 

Fig. 18b.9. Example cychc voltammograms due to (a) multi-electron transfer redox reaction two-step reduction of methyl viologen MV2++e = MV++e = MV. (b) ferrocene confined as covalently attached surface-modified electroactive species—peaks show no diffusion tail, (c) follow-up chemical reaction A and C are electroactive, C is produced from B through irreversible chemical conversion of B, and (d) electrocatalysis of hydrogen peroxide decomposition by phosphomolybdic acid adsorbed on a graphite electrode.

The redox groups can be introduced by coordination, electrostatic binding or covalent attachment to the polymer backbone. 

Figure 3.22 (a) Cyclic voltammogram of myoglobin covalently attached to a CNT forest in PBS solution under nitrogen atmosphere. The reversible redox behavior of the iron redox center is observed, (b) and (c) electrocatalytic response of Myoglobin/CNT forest electrode to oxygen and peroxide... 

A further approach to electrically wire redox enzymes by means of supramolecular structures that include CNTs as conductive elements involved the wrapping of CNTs with water-soluble polymers, for example, polyethylene imine or polyacrylic acid.54 The polymer coating enhanced the solubility of the CNTs in aqueous media, and facilitated the covalent linkage of the enzymes to the functionalized CNTs (Fig. 12.9c). The polyethylene imine-coated CNTs were covalently modified with electroactive ferrocene units, and the enzyme glucose oxidase (GOx) was covalently linked to the polymer coating. The ferrocene relay units were electrically contacted with the electrode by means of the CNTs, and the oxidized relay mediated the electron transfer from the enzyme-active center to the electrode, a process that activated the bioelectrocatalytic functions of GOx. Similar results were observed upon tethering the ferrocene units to polyacrylic acid-coated CNTs, and the covalent attachment of GOx to the modifying polymer. 

Another redox switchable system is based on dyad 21 in which 2-chloro-1,4-naphthoquinone is covalently attached to 5-dimethyl-aminonaphthalene via a non-conjugated spacer. The intrinsic fluorescence of the dansyl excited state in dyad 21 is strongly quenched, due to the intramolecular electron transfer from the excited dansyl to the adjacent quinone acceptor. However, the fluorescence can be switched on by addition of a reducing agent. Apart from chemical switching, the fluorescence of dyad 21 can also be switched electrochemically. This can be realized using a photoelec -trochemical cell, and the solution starts to fluoresce upon application of a reductive potential.31... 

Figure 11 Dependence on driving force of first-order rate constant for back electron transfer from colloidal Sn02 films to covalently attached complexes. The variations indicate that the reactions occur in the Marcus normal region. The identities of the molecular redox couples, listed from highest driving force to lowest, are, Rulll/n (5-Cl-phen)2 (phos-...

When covalently attached to electron transfer active subunits, the DHA-VHF couple can facilitate chemical and physical switching of electronic properties, as a result of photochemically induced rearrangement accompanied by a change in the redox potential. An interesting example of such a switching system is the compound containing a dihydroazulene component and a covalently attached anthraquinone moiety.1311 This system is able to act as a multimode switch, assisted by various processes such as photochromism, reversible electron transfer, and protonation-deprotonation reactions (Scheme 8). 

The approaches that have been proposed to immobilize artificial mediators include the adsorption of the redox mediator (9), the immobilization in carbon paste (75), the covalent linkage on electroinactive (75) or conducting polymer backbone (10), the covalent attachement to the enzyme structures (3) and... 

It is well known that the flavin adenine dinucleotide redox centers of many oxidases are electrically inaccessible due to the insulating effect of the surrounding protein thus, direct electron transfer from the reduced enzyme to a conventional electrode is negligible. In the present work, a variety of polymeric materials have been developed which can facilitate a flow of electrons from the flavin redox centers of oxidases to an electrode. Highly flexible siloxane and ethylene oxide polymers containing covalently attached redox moieties, such as ferrocene, are shown to be capable of rapidly re-oxidizing the reduced flavoenzyme. 

Redox polymers are electroactive polymers for which the redox centers are localized on pendent, covalently attached redox centers. The electrochemical properties of such materials depend not only on both the loading and the nature of the redox-active center but also on the type of polymer backbone. The electroactive groups are typically metal complexes, which are covalently attached to a polymer... 

Another superfamily is formed by bacterial di-heme CCP (with over 110 entries in PeroxiBase) that are periplasmic enzymes providing protection from oxidative stress. These homodimeric enzymes have a conserved tertiary structure containing two type-c hemes covalently attached to two predominantly a-helical domains via a characteristic binding motif. One heme acts as a low redox-potential center where H2O2 is reduced, and the other as a high redox-potential center that feeds electrons to the peroxidatic site from soluble electron-shuttle proteins such as cytochrome c [24]. In the crystal structure of the Geobacter sulfurreducens enzyme shown in Fig. 3.1g, the first heme appears as a bis-histidinyl-coordinated form (and... 

The possible catalytic mechanism of [FeFe] hydrogenases is not well established, since there are fewer redox states accessible to spectroscopy (e.g. by EPR), the proton-accepting base is still being debated, and the protein structure shows a larger variability. Also, the DFT modeling of the reaction cycle is more complicated because the covalently attached cubane [4Fe-4S] subcluster, which seems to play an important role in the electron shuffling, is difficult to include in the calculations. 

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